The present invention relates to a semiconductor laser, in particular, a semiconductor laser suitable for a light source for optical communication.
In recent years, in the field of optical communication, an optical communication technology for transmitting signal light emitted from a semiconductor laser through an optical fiber have been under development, and it is required to reduce the loss of the signal light in order that a photodetector on the signal receiving side efficiently can receive the signal light from the semiconductor laser that is the light source on the signal sending side. In order to reduce the loss of the signal light, a high coupling efficiency between the light emitted from the semiconductor laser and the optical fiber is required.
In general, the outgoing angle of laser light of a semiconductor laser is as wide as about 20 degrees to about 30 degrees, so that when laser light is directly coupled into an optical fiber without using an optical component such as a lens, only a low coupling efficiency such as several % can be achieved.
On the other hand, if an optical component such as a lens is inserted between a semiconductor laser and an optical fiber to focus the light, a high coupling efficiency can be achieved. However, the precision for aligning the semiconductor laser, the optical component and the optical fiber should be about 1 μm, and this need of alignment at a very high precision increases the cost due to facilities for precise processing or the like.
In order to solve these problems, the following method is under examination. The outgoing angle of the laser light of a semiconductor laser is set to about 10 degrees so as to reduce the spread of the laser light, and the laser light is coupled directly into an optical fiber. This type of a semiconductor laser that can achieve a narrow outgoing angle is disclosed in Japanese Laid-Open Patent Publication No. 2000-36638.
This known semiconductor laser will be described with reference to FIGS. 8A to 8C. FIG. 8A is a perspective view of the known semiconductor laser, and FIG. 8B is a transparent view from the top of the known semiconductor laser transparently showing the stripe structure portion for active regions. FIG. 8C is a view showing the light intensity pattern of a far-field pattern of laser light emitted from the known semiconductor laser.
As shown in FIG. 8A, in the known semiconductor laser, a buried layer 104 made of InP is formed on a substrate 101 made of InP such that a stripe structure 103 including an active layer 102 is buried. Furthermore, a part of the buried layer 104 and a part of the substrate 101 are removed to form separating grooves 105a and 105b parallel to the central line of the stripe structure 103 across the entire resonator. The stripe structure 103 includes a tapered region 106 and a parallel region 107. The laser light 108 is emitted from the end face of the tapered region 106 of the stripe structure 103.
Regarding the light propagating from the parallel region 107 to the tapered region 106 in the stripe structure 103, light confinement to the active layer 102 is reduced continuously when the light is propagating in the tapered region 106. Therefore, leakage of light from the active layer 102 to the buried layer 104 is increased, so that the spot size of the laser light 108 at the end face for light emission becomes larger than that in the parallel region 107. Such an increase of the spot size of the laser light 108 means that the outgoing angle becomes narrow.
The separating grooves 105a and 105b are formed to increase the response speed when the semiconductor laser is directly modulated. This is because the electrical capacitance is decreased by the fact that in the burrier layer 104 as a current blocking layer, a voltage is applied only to the region sandwiched by the separating grooves 105a and 105b. Thus, the response speed when the separating grooves are provided can be faster than that when the separating grooves are not provided, so that the semiconductor laser having the separating grooves 105a and 105b is effective in the case where the semiconductor laser is modulated.
However, in the laser having a tapered stripe structure including the active layer 102, the light 109 leaked from the tapered region 106 (hereinafter, referred to as “radiated light”) travels in the buried layer 104 adjacent to the active layer 102 in parallel to the substrate 101, as shown in FIG. 8B, and is reflected at the side walls of the separating grooves 105a and 105b. The reflected radiated light 110 still travels in the buried layer 104, and is emitted from the end face of the semiconductor laser to the outside together with the laser light 108 that is guided and travels in the active layer 102.
In this case, as shown in FIG. 8C, the radiated light 109 and the outgoing laser light 108 interfere with each other at the end face from which the laser light exits, so that a unimodal pattern of light intensity in the far-field pattern in the direction parallel to the substrate 101 cannot be obtained. Consequently, the utilization efficiency of the laser light with respect to the optical fiber is significantly decreased.